The invention relates to improvements in plasma etching of tungsten. More particularly, the invention provides a high temperature tungsten etching process wherein a tungsten layer is selectively etched using a chlorine and oxygen plasma.
Vacuum processing chambers are generally used for etching of materials on substrates by supplying process gas to the vacuum chamber and application of an RF field to the gas. Examples of parallel plate, transformer coupled plasma (TCP(trademark), also called ICP), and electron-cyclotron resonance (ECR) reactors are disclosed in commonly owned U.S. Pat. Nos. 4,340,462; 4,948,458; and 5,200,232. The substrates are held in place within the vacuum chamber during processing by substrate holders. Substrate holders in the form of an electrode can supply radiofrequency (RF) power into the chamber, as disclosed in U.S. Pat. No. 4,579,618. Plasma processing systems wherein an antenna coupled to a radiofrequency (RF) source energizes gas into a plasma state within a process chamber are disclosed in U.S. Pat. Nos. 4,948,458; 5,198,718; 5,241,245; 5,304,279; and 5,401,350. In such systems, the antenna is located outside the process chamber and the RF energy is supplied into the chamber through a dielectric window. Such processing systems can be used for a variety of semiconductor processing applications such as etching, deposition, resist stripping, etc.
Plasma etching of tungsten is disclosed in U.S. Pat. Nos. 4,713,141; 4,842,676; 4,842,687; 5,259,923; and 5,882,992. Of these, the ""141 patent discloses etching tungsten overlying a dielectric layer using a mixture of SF6 and Cl2 at a flow rate of 100 to 200 sccm, pressure of 200 to 400 mTorr, an electrode gap in a parallel plate plasma etcher of 0.6 to 1 cm, an electrode temperature of 20xc2x0 C., and RF power of 150 to 275 watts supplied to the electrode. The ""676 and ""687 patents disclose selective etching of tungsten at ambient temperature with gas mixtures such as SF6, HBr and a source of hydrocarbons. The background of the ""923 patent states that fluorine containing gases such as SF6 or a mixture of SF6 and CCl4 or Cl2 have been used to etch tungsten but the selection ratio of such gases against silicon or silicon oxide is not sufficient. The ""992 patent discloses etching of tungsten over chromium with a mixture of SF6 and CHCl3 followed by stripping the photoresist mask using a low temperature ( less than 100xc2x0 C.) plasma etch in O2.
During etching of tungsten with fluorine, the reaction of tungsten with the fluorine atom forms volatile WF6. However, because fluorine also attacks silicon oxide, etching of tungsten over silicon oxide with desired selectivity cannot be achieved by conventional plasma etch processes. Accordingly, it would be desirable for a plasma etch process to be capable of providing a desired degree of selectivity between the etch rate of a tungsten layer and an underlying silicon dioxide layer.
The invention provides a method of etching a tungsten film, comprising the steps of supporting a semiconductor substrate having a tungsten film thereon on a substrate support in an interior of a plasma etcher, supplying process gas to the interior of the plasma etcher, energizing the process gas into a plasma state, etching the tungsten film by exposing the substrate to the plasma, and heating the substrate to a temperature of at least 100xc2x0 C. during the etching step.
During the process, the substrate can be heated to at least 200xc2x0 C., preferably 300xc2x0 C. to 400xc2x0 C. The etching step can include a low temperature main etch below 100xc2x0 C. followed by a high temperature overetch above 100xc2x0 C., the process gas including a fluorine containing gas during the main etch and a chlorine containing gas during the overetch. Preferably, the overetch is carried out using Cl2 and O2 as the process gas. The tungsten film preferably overlies a dielectric film such as a silicon oxide film having a thickness of 200 xc3x85 or less. The tungsten film can be a pure tungsten film covered with a patterned resist film or patterned hard mask such as silicon dioxide or silicon nitride, the hard mask being preferred during the high temperature tungsten etching. For example, the hard mask can be an antireflective coating and the antireflective coating (ARC) can be covered with a patterned resist film, the method including plasma etching exposed portions of the ARC. Preferably, a radio frequency bias is applied to the substrate during the etching step, e.g., a radio frequency bias of 50 to 300 watts can be applied to the substrate during the etching step while maintaining the interior of the plasma etcher at a vacuum of less than 20 mTorr, preferably less than 10 mTorr. The plasma is preferably generated by an antenna which inductively couples radio frequency power into the interior of the plasma etcher, the antenna being supplied radio frequency power by a radio frequency source driven at a power level of 1000 watts or less, e.g., 300 to 700 watts. If desired, the process gas during the high temperature etch can include Cl2 and O2 or HBr and Cl2 and the process gas during the low temperature etch can include SF6, N2 and Cl2; SF6 and N2; or CF4, N2 and Cl2.